1. Field of the Invention
The present invention relates to a liquid crystal composition, a liquid crystal device, a reflective display material and a light modulating material, and particularly to a liquid crystal composition, a liquid crystal device, a reflective display material and a light modulating material which can be suitably used for a guest-host system.
2. Description of the Related Art
In a liquid crystal device of a guest-host system, images are displayed by changing the light absorption state of the liquid crystal, based on the change in orientation of a dichroic dye dissolved in the liquid crystal in response to the movement of the liquid crystal due to an electric field. Therefore, the liquid crystal device of a guest-host system is expected to achieve a high-speed response and an excellent contrast ratio.
The guest-host system has been discussed in various literature (for example, refer to “Handbook of Liquid Crystals”, Vol. 2A, Chapters 3 and 4, pp. 257-302, written by B. Bahadur, edited by D. Demus, J. Goodby, G. W. Gray, H. W. Spiess, and V. Vill, published by Wiley-VCH (1998)). There has been a demand that dichroic dyes used in a guest-host liquid crystal device have a proper absorption property, a high order parameter, and a high degree pf solubility in a host liquid crystal, and there have been reports on azo dyes, anthraquinone dyes and the like. Among these, from the viewpoint of the resistance to heat and light, anthraquinone dyes have been intensely studied.
However, due to the low degree of molar absorption coefficient thereof, anthraquinone dyes have to be dissolved in a liquid crystal in an extremely large amount in order to increase color density, thereby requiring a high degree of solubility in the host liquid crystal. In order to overcome the above problems, methods of improving the solubility of a dichroic dye have been reported, in which the dichroic dye is provided with a substituent including a cyclohexane ring and having a structure similar to a host liquid crystal (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 58-57488 and 2003-113378).
Further, in order to exhibit a highly pure black color, it is necessary to mix at least three kinds of dyes having different light absorption wavelengths, and it is also necessary to control the maximum absorption wavelength and the absorption waveform of these dyes, so the structure of the dyes is largely restricted. Consequently, very few dyes have been known which can meet the above requirements when combined with other dyes, even if they can satisfy requirements such as order parameter, light fastness and solubility in a liquid crystal, when used for a single color (for example, see JP-A Nos. 2000-336366 and 62-277471).
It is generally known that when a substituent that causes steric hindrance is introduced into a dye molecule for the purpose of enhancing the solubility, the order parameter is lowered. However, the present inventors have found that by mixing plural kinds of dyes having a specific substituent, such an unexpected result as an increase in the order parameter can be achieved.